Biofilms are complex communities of microorganisms, comprised either of a single or multiple species. Over the past few decades, there has been a growing realization that bacteria in most environments are not found in a unicellular, planktonic existence such as those typically studied in the laboratory, but exist predominantly in multi-cellular surface attached communities called biofilms. This realization has spurred much research into the physical and chemical properties of biofilms, their morphology, and the mechanism of their development.
The transition from the planktonic mode of existence to a biofilm is a regulated developmental process. This biofilm community has a number of distinct characteristics including the production of exopolysaccharides, the formation of chemical and pH gradients, a marked degree of structural heterogeneity, and the development of high level resistance to a variety of antimicrobial agents.
It has been shown that biofilm grown cells can become 10–1000× more resistant to the effects of antimicrobial agents than their planktonic counterparts. This characteristic of biofilms makes them extremely difficult to control in both medical and industrial settings. Traditional antibiotic therapies can eliminate planktonic bacteria, but organisms growing in a biofilm survive treatment and can eventually regrow once antibiotics are discontinued. The levels of antibiotic required to eliminate biofilm bacteria often cannot be achieved in the patient or are toxic. Therefore, biofilm-based infections can become chronic with the only recourse being removal of the contaminated surface.
The formation of biofilms can have serious negative consequences in medical, industrial, and natural settings, resulting in high costs both in human health and economic terms. Biofilm-associated infections extend hospital stays an average of about three days and it is estimated that up to 65% of nosocomial infections are biofilm-based with an associated treatment cost in excess of a billion dollars per year. In clinical settings, biofilms can form on a variety of surfaces. Biofilms formed on indwelling medical devices serve as a reservoir of bacteria that can be shed into the body, leading to a chronic systemic infection. Indeed, up to 82% of nosocomial bacteremias are the result of bacterial contamination of intravascular catheterizations. Examples of biofilms include oral microbes on teeth, chronic Pseudomonas aeruginosa infections in the lungs of cystic fibrosis patients and bacterial contaminants on medical devices such as pacemakers and catheters. Biofilms can form in almost any hydrated environment that has the proper nutrient conditions, and can develop on a wide variety of abiotic (both hydrophobic and hydrophilic) and biotic (e.g., eukaryotic cells) surfaces.
The formation of biofilms is an important aspect of normal development for many microbial species. The mechanisms responsible for the increased biofilm resistance, however, are not well understood. It has been suggested that the exopolysaccharide matrix that surrounds the cells in the biofilm prevent diffusion of antimicrobial agents through the biofilm, thus preventing access of the agent to the cells. While this may be the case for some antimicrobial agents, for many others it has been shown that antimicrobial agents can penetrate the biofilm matrix but are still unable to kill cells in the biofilm. It has also been suggested that cells within the biofilm grow slowly in response to nutrient deprivation and perhaps some form of stress. Therefore, antimicrobial agents that only act on actively dividing bacterial cells would be non-functional in this sort of environment. While a number of studies support the idea that slowed growth rate can explain some aspects of biofilm-related resistance, other studies have suggested that the full extent of resistance cannot be accounted for by this mechanism. Finally, while it has been suggested that quorum sensing is involved in resistance to antimicrobial agents, it is not clear what role, if any, this system plays in biofilm-related antimicrobial resistance. To date, none of the models put forward adequately explain the level of resistance to biocides attained by cells in a biofilm.
There is emerging evidence that the transition from planktonically growing bacteria to life in a biofilm requires a genetic program that responds to a variety of environmental cues. It is possible that the development of biofilm-related antibiotic resistance is also a regulated event, and taken together with the marked biochemical and physiological heterogeneity of biofilms, the induction of a biofilm-related resistance phenotype may occur within distinct regions of the biofilm. The subset of biocide-resistant cells in the biofilm is referred to as “persistors”. The term persistor refers to the fact that not all of the cells within the biofilm resist killing by antimicrobial agents resulting in the survival of a small population of very resistant cells after antibiotic treatment.
There exists a strong need to discover methods and compositions that will inhibit biofilm formation and overcome their resistance mechanisms.